In a pure series hybrid a gas engine acts a generator which charges a battery, which drives an electric motor, which provides power to the wheels.

In parallel hybrids, both the gas motor and the electric motor provide traction, sometimes simultaneously.

Parallel is more efficient if you're depending on gas often, as every conversion has losses. Toyota mostly sells hybrids you can't plug in, and the ones you can plug in are variants of the ones you can't, so they all share the same drivetrain.